Arrangements for electrothermally atomizing analysis specimens are known as graphite furnaces in the area of atomic absorption spectroscopy. The graphite furnaces always essentially comprise a furnace body which is disposed in a protective gas atmosphere between two electrodes as disclosed in German patent publication 2,413,782. The furnace body is often configured as a graphite tube and is referred to as such and the electrodes are configured as graphite housing parts. The electrodes are usually held in water-cooled metal jackets and are connected via these jackets to a controllable current source. The furnace body, and therefore the specimen to be analyzed, is electrothermally heated with the aid of a time-controlled high current conducted through the cross section of the furnace body. The specimen to be analyzed is converted into free atoms as completely as possible in the inner chamber of the furnace body. The free atoms can be optically detected. The effectiveness of the atomization is essentially determined by the speed of the temperature heating process and the quality of the transfer to an isotropic temperature distribution in the interior chamber of the furnace body which is necessary for the atoms to be determined. The process of generating and of the residence (breakdown) of the free atoms is generally known as the atomizing phase and includes a typical time duration of 1 to 10 seconds in dependence upon the element to be determined and on the quantity of the analyte. The atomizing temperatures necessary for the specific element extend over a range of 800.degree. to 2800.degree. C. The temperature is the most essential parameter determining the quality criteria, the detection capability and the reproducibility for the atomization process with the electrothermally heated furnace body. Generally, an isotropic temperature distribution in the entire furnace body cannot be assumed. A furnace body temperature at a specific time point requires the availability of a specific electric power. Characteristics of the furnace body, on the one hand, such as material characteristics, mass and geometric form as well as, on the other hand, characteristics of the graphite oven sleeve in contact with the furnace body have a complex influence on the particular furnace body temperature which is adjusted and finally on the electric power necessary for generating this temperature. The characteristics of the graphite oven sleeve include the contacting and heat transfer of the cooling, radiation feedback of the surrounding inner surface of the graphite furnace and the means for conducting the protective gas.
The furnace body temperature is essentially determined by the difference of heat quantity from the heat generated by the supplied current and the heat losses flowing away via the furnace contacts in the range of room temperature to approximately 1000.degree. C. On the other hand, above 1500.degree. C. (increasing proportionally to the fourth power of the temperature), the difference of the generated heat and the loss quotient of heat radiating from the furnace body and the heat radiated back from the interior of the furnace is more significant.
Graphite furnaces have been known in the state of the art for approximately 20 years and have a tube-shaped furnace body with the following dimensions: 20 to 40 mm length, 3 to 6 mm inner diameter and 0.5 to 1.5 mm wall thickness. These graphite furnaces are known as graphite tubes and are usually held at their ends in cooled ring-shaped contacts. The graphite tubes are flushed within and without with flows of protective gas and are enclosed in furnace housing parts defining a hollow chamber. The housing parts are electrically insulated with respect to each other. The delivery channel for the specimen to be analyzed and a gap-like interface in the furnace housing are the only openings of this kind for a half-closed graphite furnace arrangement which is known also as a Massmann furnace. A significant disadvantage of this arrangement is that a temperature gradient of several hundred degrees is formed by the heat flowing off via the furnace body ends. This characteristic leads to the deterioration of the effectiveness of the atomization and to analytic disturbances because of condensation processes of the analyte components previously vaporized in the heating phase at the cooled furnace body ends. Furnace arrangements of this kind are disclosed, for example, in published German patent application 2,413,782.
Graphite furnace arrangements of the Massmann type having a more uniform temperature distribution along the tube axis as compared to the previously described arrangement are obtainable by means of a cross section profile of the tube wall thickness. Different variations are known wherein the tube body exhibits a reduced electrical conductivity especially in the vicinity of its contacts. The temperature drop because of a flow of heat to the contacts essentially remains but the zone of isotropic temperature relationships starting at the tube center is expanded. Examples of the foregoing are provided in German patent publications 3,228,245; 3,442,073; and 2,148,777.
It is furthermore known that an isotropic temperature distribution along the furnace body axis is then obtained if the heating current is conducted in the peripheral direction of the furnace body transversely to the tube axis. This temperature distribution is better suited to the analytic requirements of the electrothermal atomization. Two so-called open graphite furnace arrangements are disclosed in U.S. Pat. 4,407,582 wherein one pair or two pairs of bifurcated contact pieces are in contact engagement radially on mutually opposite sides of the tube-shaped furnace body. Significant disadvantages of these arrangements are: the poor contact at the hot outer surface of the furnace body, the isotropic temperature relationships only in the region of the contact locations and the missing protective sleeve in the form of a furnace housing as protection against effects of atmospheric oxygen.
Configurations of a tube-shaped graphite furnace body having diagonally opposite-lying contact pieces are known as "Frech-Hutsch cuvettes". The contact pieces are formed on the outer surface on the longitudinal sides of the tube axis. In this connection, reference may be made to published German utility model registration 8,714,670 and German patent publication 3,534,417.
Different geometric modifications for influencing the furnace body temperature are described.
These configurations are only useful to a limited extent since no solutions are offered for the configuration of graphite furnace arrangements on the basis of transversely heated cuvettes for the complex exchange effect problems which occur thereby. The described geometric variations have no isotropic current density and therefore no isotropic temperature relationships over the length of the tube outer surface.
The configuration of a graphite furnace arrangement for a tube-shaped, transversely-heated furnace body having diametrically opposite lying contact pieces is disclosed, for example, in European patent publications 0,350,722; 0,303,134; and, 0,311,761 as well as in published German patent publications 3,802,968 and 3,735,013. A furnace body is described having a geometry which is complicated to manufacture and therefore is of high cost to the user as a part subject to wear. The furnace body is held between two large cylindrical contacts defining a cylindrical hollow chamber so that it can be flushed with a protective gas flow and is enclosed by the two contacts which conjointly define an annular gap therebetween. The heat balance and heat distribution of the tube-shaped furnace body part of this graphite furnace arrangement are not isotropic along the length of the tube axis and especially during the usual heating time of approximately 1 second of the atomizing step and the holding phase of several seconds which follows. The contact pieces have a trapezoidal shape when viewed in plan and are formed on the tube-shaped furnace body. The contact pieces have several breakthroughs and constrictions on the longitudinal side of the tube outer surface so that a constant current density distribution is not provided and therefore also a constant generation of heat and removal of heat over the tube length are not provided. Furthermore, the circularly-shaped configuration of the electrode inner wall over the longitudinal tube axis leads to nonuniform heat losses because of nonuniform interaction of radiation between furnace body surface and inner chamber wall. This increases with temperatures above 1500.degree. C. with the loss components on the tube-body ends being significantly higher than at the tube center.
The contact projections are large and are held relatively cold. These projections have a large heat capacity so that the actual furnace body can only then maintain a temperature level obtained at the end of the heating phase when the heat loss which occurs immediately because of the thermal balancing processes is compensated by a complex control algorithm for the heat capacity. Otherwise, considerable losses of analyte atoms would occur because of the immediate drop of the furnace body temperature.
The contact surfaces are configured to be small with respect to area and are between the two contacts and the furnace body contact pieces and lead to local overheating and power losses and overall to an unstable contact and temperature performance. This is caused by the heating rate required for an effective atomization which is up to 2000 kelvin per second and by the necessary current intensities of approximately 1000 ampere (effective).